MgO thin films have been grown on Si(100) by atomic layer deposition using bis(cyclopentadienyl)magnesium and water as precursors. The effect of rapid thermal annealing (RTA) on thermal stability and interfacial properties of MgO films is investigated by X-ray photoelectron spectroscopy (XPS). XPS results indicate that the as-deposited MgO film is quite reactive to the atmospheric moisture and carbon dioxide and has a sharp interface with the substrate. The spectra of samples annealed at 600 degrees C show that the Mg-OH groups have been turned into MgO. RTA also induces the formation of magnesium silicate, whose content increases with increasing annealing temperature.